1. Field of the Invention
The present invention is on a dispersion slope equalizer that simultaneously recovers waveforms of wavelength division multiplexed (WDM) signals distorted by dispersion characteristic (that is; group delay is different at each wavelength) in an optical fiber.
2. Description of the Related Art
Today, the investigation to achieve a large capacity optical transmission systems is being actively carried out utilizing a dispersion shifted fiber (DSF) having zero dispersion or a non-zero dispersion shifted fiber (NZ-DSF) having several psec/nm/km dispersion at wavelength λ=1.55 μm band.
However, when WDM transmission using DSF's is carried out in order to realize large capacity networks, second-order dispersion (derivative of group delay with respect to wavelength) is different at each wavelength channel as shown in FIG. 20 (relative delay time characteristics of DSF). This dispersion difference is caused by dispersion slope, namely, third-order dispersion (derivative of second-order dispersion with respect to wavelength), whose typical value is about 0.07 psec/nm2/km. As a result, the signal pulse at each wavelength channel is differently distorted, which limits the transmission capacity or propagation distance (distance between repeaters). This is also the same as when NZ-DSF is used.
Up to now, configuration in FIG. 21 has been known as a dispersion slope equalizer, which recovers the distorted waveforms of WDM signals by the dispersion in fibers. In FIG. 21, dispersion compensating fiber (DCF) 44 is set behind fiber transmission line 43. In this configuration, dispersion characteristic in the fiber transmission line can be compensated by the DCF, whose dispersion has the same absolute value with opposite sign to that of the fiber transmission line.
However, the prior art dispersion slope equalizer having the foregoing configuration has following problems:    (1) by utilizing the foregoing configuration, it is possible to compensate for dispersion slope of a 1.3 μm band zero dispersion fiber (that is; single mode fiber (SMF)) at 1.55 μm band, but it is impossible to compensate for dispersion slope of DSF or NZ-DSF at 1.55 μm band,    (2) configuration is complicated and the size is large, and    (3) compensation value is fixed, and it is impossible to compensate for the dispersion of various fiber transmission lines having various dispersion values and variation of dispersion value caused by the temperature change or the like by utilizing the foregoing configuration.